Conventionally, in electric appliances such as microwave ovens, while earth terminals are provided with main bodies of these microwave ovens, the earth terminals are grounded during operations in order to avoid such a risk that users may suffer from electric shock failures. Also, there are some microwave ovens equipped with earth leakage cut off functions, assuming the case that earth terminals provided with main bodies of microwave ovens are not firmly grounded.
FIG. 11 shows a circuit diagram of the above-described microwave oven. As shown in FIG. 11, the microwave oven is arranged by a magnetron 104, a control board 106, a relay 107, and a zero phase current transformer 108. The magnetron 104 oscillates with a power supply that comprises a high voltage transformer 101, a capacitor 102, and a diode 103. The control board 106 operates receiving electric power supplied by a low voltage transformer 105. The relay 107 opens and closes a connection between a power supply and a high voltage transformer 101. The zero phase current transformer 108 is provided on a power supply line between the power supply and the relay 107. Since the relay 107 operates receiving a control signal supplied from the control board 106 so as to supply the electric power to the high voltage transformer 101, the magnetron 104 oscillates to heat food. Also, the control board 106 monitors an unbalance current detected by the zero phase current transformer 108 in order to detect earth leakage. When the earth leakage is detected, the control board 106 immediately opens the relay 107 in order to cut off an earth leakage current (refer to patent publication 1).
Patent Publication 1: JP-A-58-175282